The present invention relates to slurry feeding apparatus and method for use in a chemical/mechanical polishing (CMP) process of a wafer.
In recent years, the surface of a semiconductor wafer is often planarized by a CMP technique to ensure sufficient uniformity for an interlevel dielectric film, for example, during the manufacturing process of transistors on the substrate. The CMP process is performed using a kind of slurry, where fumed or colloidal silica is dispersed as abrasive grains in an alkaline solution of ammonium, for example.
FIG. 8 illustrates a cross section of a known (polishing) slurry feeding apparatus F1 as disclosed in Japanese Laid-Open Publication No. 10-15822.
As shown in FIG. 8, the slurry feeding apparatus F1 includes tank 101, delivery pipe 102 with a pump 104, flow rate control valve 103, feeding nozzle 110 and stirrer 106. Polishing slurry 109 is stored in the tank 101 and delivered through the delivery pipe 102 from the tank 101 to a CMP polisher (not shown). The flow rate control valve 103 is provided in the middle of the pipe 102 downstream of the pump 104. The feeding nozzle 110 is attached to the end of the pipe 102 for dripping the slurry 109 onto a polishing pad (not shown) of the polisher. And the stirrer 106 with a propeller is used for stirring the slurry 109. A circulation pipe 105 is further provided as a branch from the delivery pipe 102 upstream of the valve 103 to circulate the slurry  109 by feeding the slurry 109 back to the tank 101 therethrough. A heater 107 is further provided on the bottom of the tank 101 to regulate the temperature of the slurry 109 within the tank 101. The temperature of the heater 107 is in turn regulated by a heater temperature controller 108. In polishing a wafer, the opening of the valve 103 is adjusted and a predetermined amount of the slurry 109 is sucked up from the tank 101 using the pump 104 and then dripped onto the polishing pad through the feeding nozzle 110. The remainder of the slurry 109 is recovered to the tank 101 through the circulation pipe 105. On the other hand, while the polishing process is not performed, the valve 103 is closed and all the slurry 109 is recovered to the tank 101, thereby circulating the slurry 109 without delivering it.
As for colloidal silica, the primary grains thereof have a tiny size of 20 to 30 nm. But in the polishing slurry 109, a certain number of primary silica grains coagulate to form secondary grains with a size of 100 to 200 nm. As for fumed silica on the other hand, the grain size thereof is 100 to 200 nm from the beginning (i.e., when they are prepared). Thus, it is generally believed that these secondary grains with a grain size of 100 to 200 nm actually contribute to the polishing process.
Nevertheless, if an excessive number of abrasive grains coagulate together to form grains with a size as large as about 500 nm or more, then micro-scratches are possibly made on the object being polished.
Thus, the conventional slurry feeding apparatus F1 always circulates the polishing slurry 109 and stirs the slurry 109 up with the propeller, thereby suppressing the sedimentation and coagulation of the abrasive grains in the slurry 109.
FIG. 10 illustrates a cross section of a coupling generally provided for the piping where the slurry flows in a conventional slurry feeding apparatus. By using couplings in various shapes for the corner or linear portions, piping can be formed in a complicated shape and the cross-sectional area of the piping and the overall size of the slurry feeding apparatus can be both reduced.
It is known that the excessively promoted coagulation of the abrasive grains (e.g., with a grain size of more than about 500 nm) not only causes micro-scratches on the object being polished but also decreases the polishing rate.
FIG. 9 is a graph illustrating, in comparison, respective polishing rates of Slurry 1 and 2 with mutually different concentrations of solid content (abrasive grains) in accordance with results of experiments carried out by the present inventors. As can be seen from FIG. 9, although the solid content concentration of Slurry 1 is only 1% lower than that of Slurry 2, the polishing rate attained by Slurry 1 is considerably lower than that attained by Slurry 2. Such a decrease in solid content concentration could result from the sedimentation of abrasive grains with an excessively increased size in the tank. Accordingly, it is critical to prevent the size of abrasive grains from increasing excessively in order to obtain an appropriate polishing rate.
To suppress the coagulation of abrasive grains, the conventional slurry feeding apparatus has the following draw-backs.
Firstly, the increase in size of abrasive grains in the slurry 109 cannot be suppressed sufficiently only by stirring the slurry 109 up using the stirrer 106 with a propeller as shown in FIG. 8.
Secondly, the slurry 109 is likely to form puddles here and there in the regions Rg of the coupling where two pipes of the piping are joined together in the slurry feeding apparatus F1. This is because there are many gaps and level differences between these pipes in the region Rg as shown in FIG. 10. As a result, the excessive coagulation of the abrasive grains is possibly promoted.
Thirdly, the solidified contents of the slurry 109 are likely to deposit on the inner walls of the tank 101 as the level of the slurry solution changes in the tank 101. And the solidified slurry 109 once deposited will collapse within the tank 101 to increase the size of the grains coagulated.
Since the size of the abrasive grains is excessively increased in this manner, the micro-scratches are made on the object being polished and the polishing rate thereof decreases or becomes inconstant.